As per the title, I want to fit fit one, to reduce the parachute effect, but I have a problem.

The chassis has two tubes either side which come out from the bottom of the chassis in front of the rear wheels, and straight, then bending upwards to
meet the rail around the bottom of the rear tub. This means that if I did fit a diffuser, it would not be able to follow the edge of the rear tub, and
there would be a long triangular gap each side, with the diffuser hanging down from the rear of the tub by about 100 mm

There is a trianle gap down each side, I have used 5mm spacers at the rear fixing points but they aren't essential. Under the car, it fastens
flat to those chassis rails you mention, then slope up to the rear.